What’s tactical voting?

If you support a candidate that you think won’t get elected under FPTP, you can either choose to “waste” your vote by voting for them anyway, or vote for someone who seems most similar to your preferred candidate. This usually happens because you fear that by “wasting” your vote, you could allow someone you really don’t want to be elected.

Here’s how it works for FPTP:

What effect does tactical voting have?

Because voters have to guess how everyone else will vote, the election result can be drastically affected by outside influences:

The media can influence who voters think will be the top two candidates, and even affect those who distrust the media, because they have to guess how people who do trust the media will vote. The top two candidates predicted by the media cause tactical voting by supporters of any other candidate.
(Complicated, huh?)

An unproven candidate, like a new party member where an old hand has retired, might seem to be unlikely to win, so tactical voting means people don’t vote for them, and then their track record is of poor turnout.
(Self-fulfilling prophecy time.)

Campaigns can be based entirely on who to vote against, where a party might tell its supporters to vote tactically for someone else to ensure another candidate doesn’t get elected.
(Turkeys cheering Christmas, now.)

If enough voters are voting tactically, FPTP effectively becomes a preferential voting system like AV, with the first rounds being carried out in the media.
(And finally, we hand control of our democracy to the media tycoons.)

Tactical voting has bad consequences for democracy, and is just one of the bad sides of minority election.

How does AV prevent tactical voting?

In AV, you can vote for your favourite candidate with your first preference, secure in the knowledge that, if your favourite is eliminated, you won’t have harmed the chances of your second (or later) preference. Your vote still counts, though not for the first candidate you had chosen. Here’s the same process as above, but under AV:

It’s also much more difficult to game the system with AV, making it harder to predict which tactical voting might help which candidates.

Both these facts discourage tactical voting under AV, and can almost eliminate its damaging effects.